In this article
“Photograph: 3.5 cm × 4.5 cm, 20–50 KB, JPG, 200 DPI.” Instructions like this mix four different kinds of measurement — and that’s why so many uploads get rejected. Once you know what each one means, meeting any requirement becomes easy.
The four measurements
| Term | What it measures | Example |
|---|---|---|
| Pixels | The image’s dimensions on screen | 413 × 531 px |
| cm / mm / inches | The printed size on paper | 3.5 × 4.5 cm |
| DPI (dots per inch) | How many pixels are printed per inch | 300 DPI |
| KB / MB | How much storage the file takes | 45 KB |
The key insight: pixels and KB are what matter for online uploads. Physical size and DPI only matter when the photo is printed — or when a form describes the size that way.
Converting physical size to pixels
Pixels = millimetres ÷ 25.4 × DPI
(25.4 is the number of millimetres in an inch.)
3.5 cm × 4.5 cm at 300 DPI
Width: 35 ÷ 25.4 × 300 ≈ 413 px
Height: 45 ÷ 25.4 × 300 ≈ 531 px
| Physical size | at 200 DPI | at 300 DPI |
|---|---|---|
| 35 × 45 mm | 276 × 354 px | 413 × 531 px |
| 2 × 2 inches (51 × 51 mm) | 400 × 400 px | 600 × 600 px |
| 3.5 × 2.5 cm (signature box) | 276 × 197 px | 413 × 295 px |
If a form gives pixel dimensions directly, always use those — no conversion needed.
The DPI myth
Many people try to meet a KB limit by changing DPI from 300 to 72. It doesn’t work: DPI is just a number stored in the file telling a printer how to scale it. The file still has the same pixels, so its size barely changes.
What actually reduces file size:
- Fewer pixels (resizing)
- Stronger compression (lower JPG quality)
Step by step: meeting a photo requirement
- Read the rules carefully — dimensions, KB range, format, background colour, and how recent the photo must be.
- Crop to the right shape (aspect ratio). A 35 × 45 mm photo has a ratio of 7:9 — taller than wide. Keep your face centred and fill the frame as required.
- Resize to the exact pixel dimensions.
- Save as JPG.
- Compress until the file is inside the KB range — not too big, not too small.
- Check the photo looks clear and natural at 100% zoom.
Aim slightly inside the KB range — for “20–50 KB”, target 30–45 KB — so differences in how portals count kilobytes don't trip you up.
Photo quality rules that often apply
Requirements vary, but many forms ask for:
- A recent colour photo
- A plain white or light background
- Face clearly visible, looking straight at the camera, eyes open
- No caps or sunglasses; spectacles allowed only without glare (if at all)
- Even lighting with no shadows on the face or background
Passport and visa photos usually have stricter rules than exam forms. Always follow the official guidelines for the specific application.
Signatures
Signatures are usually wider than tall and need a smaller file:
- Sign with a black pen on white paper.
- Crop tightly, resize to the requested dimensions, and compress — signatures with lots of white space compress to a few KB easily.
Do it in two steps
Use our Resize Image tool to set the exact pixel dimensions (with aspect-ratio lock or exact sizes), then our Compress Image tool to bring the file into the required KB range. Both run entirely in your browser, so your photo stays private.
Free toolTry the Resize ImageChange an image’s width and height in pixels or by percentage.Frequently asked questions
My form says 3.5 cm × 4.5 cm. How many pixels is that?
It depends on the DPI the form expects. At 300 DPI it's about 413 × 531 pixels; at 200 DPI about 276 × 354 pixels. If the form gives pixel dimensions, use those instead.
Will changing DPI make my file smaller?
No. DPI is just a label stored in the file. Only pixel dimensions and compression change the file size.
Can I take the photo myself?
For online exam forms, a clear self-taken photo usually works if it meets the rules. For passports and visas, many authorities have strict requirements — check the official guidelines or visit a studio.
Written by the Toolsopedia team. Spotted an error? Tell us and we’ll fix it.